Electrocatalyst parallel reactions

However, on ft-based electrocatalysts, the reaction can follow parallel pathways which can, in principle, be formulated as follows [20] ... 

Here, M represents the electronically conducting electrode material (e.g.. Ft) that is not involved in the overall reaction and plays the role of an electrocatalyst for the reaction. The last intermediate step occurs in two identical consecutive steps since electron transfer occurs by quantum mechanical tunneling, which involves only one electron transfer at a time. When multistep reactions take place, there is the possibility of parallel-intermediate steps. The parallel-step reactions could lead to the same final product or to different products. Direct electro-oxidation of organic fuels, such as hydrocarbons or alcohols, in a fuel cell exhibits this behavior. For instance, in the case of methanol, a six-electron transfer, complete oxidation to carbon dioxide can occur consecutively in six or more consecutive steps. In addition, partially oxidized reaction products could arise, producing formaldehyde and formic acid in parallel reactions. These, in turn, could then be oxidized to methanol. 

However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). 

The adsorption of alcohols, aldehydes, and carbon oxides on metal electrocatalysts has been extensively studied because of the significance of their oxidation reactions for electrochemical energy generation (7,9,81,195). Particular attention has been payed to the surface intermediates of methanol oxidation on platinum. At least two adsorption states have been assigned to methanol, a weak one possibly associated with physisorption (196) and one or more states arising from dissociative strong adsorption of the reactant (797, 198). Breiter (799) proposed a parallel scheme for methanol oxidation... 

The efifect of electrocatalyst and operating conditions on selectivity were examined recently for the reduction of olefinic halides to saturated halides or cleavage products (hydrogenation versus hydrogenolysis) 31. These two steps proceed in parallel at different rates on various electrocatalysts. Thus, the ability for double bond reduction decreases in the order Pd Ru > Ag > Pt, although the overall rate is about the same on Pd and Pt (cf. Table VI). Figure 24 shows the extensive variation of reaction specificity with cathode potential as well as the smaller effect of electrolyte concentration. Similar behavior is exhibited by other halides and electrodes 31. ... 

The parallels between Schemes 1 and 2 illustrates that a knowledge of stoichiometric reactions can be utilized in the design of electrocatalysts. In particular, the use of electrochemical reductions to generate metal hydride complexes could result in a number of different types of catalytic reductions depending on the nature of the substrate. 

Analyzing Figure 4.24 a number of intriguing questions emerge (i) is the faradaic dual pathway mechanism completely parallel or under certain conditions (e.g., catalyst or co-catalyst, potential, temperature) an interconversion between the labile and strongly adsorbed intermediates, [Fi] and [Fn] respectively, can occur (as exemplified by Equation 4.20) (ii) what is the contributions of the non-faradaic, thermocatalytic, pathway during formic acid electrooxidation on various electrocatalysts (e.g., [Hn] = [Fn] = COad) , (iii) what catalyst compositions favor the pathway of least kinetic resistance to oxidation (i.e., via the intermediate [Fi]) , (iv) what is the role of Had and under what conditions OH d plays a role in the overall reaction scheme ... 

A non-uniform electrode was introduced in 1989 [73]. In this electrode, the electrocatalyst concentration increased along the direction parallel to the electrode substrate. It is expected that the effect of increased catalyst loading in the direction of gas flow could balance the effect of diminishing reactants in the gas stream. In this way, the reaction rate could be substantially uniform across the electrode surface. Prasanna et al. [74] employed the catalyst-gradient method for single fuel cell fabrication, and found that it was an effective way to reduce Pt loading without... 

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